The present invention relates to the field of radiation therapy. Radiation therapy is the therapeutic use of ionizing radiation. Ionizing x-rays are produced by a device called a linear accelerator. Ionizing gamma rays can also be harnessed using nuclear material. These ionizing radiations are directed into a patient's body to treat both benign and malignant highly proliferating cells or undesired tissues. The success of such a treatment is highly dependent on the distribution of the radiation within the patient. Sub-optimal treatment delivery techniques can result in sub-optimal results, patient injury or even death. For this reason, much effort has been put forth by practitioners to create new equipment and algorithms to enhance the therapeutic ratio (benefit over negative side effects) of the radiotherapy treatment and increase the options available to patients
Modern computational capabilities and modern equipment have enabled practitioners to create much more complex treatment plans that enhance the therapeutic ratio of the radiotherapy treatment. These advancements have simultaneously increased the chances of errors being undetected. For this reason, there is an increased need for new devices capable of verifying the planned dose distributions in a meaningful way.
In modern practice, it is common for practitioners to perform a verification measurement of the patient-specific radiotherapy plan on a patient surrogate. These surrogates are typically referred to as “radiotherapy phantoms” or just “phantoms”. A radiotherapy phantom is a radiation attenuating medium such as water, PMMA (polymethyl methacrylate), metal, wood, gel, wax, plastic or any material having a radiation attenuation similar to water. The phantom will often contain or support a method for the detection and recording of the radiation dose distribution. A computer algorithm is used to predict the dose distribution within the phantom that would be produced if one were to deliver the patient plan to the phantom. During such a measurement a practitioner may place the phantom into a treatment room aligned in a known geometry within the treatment machine. Once the phantom is in place, the radiotherapy treatment plan is delivered. The measured dose is then compared to that which was predicted by the computer algorithm. If the result is within the acceptable criteria outlined by the institution, then the plan will proceed. If it does not meet criteria, the plan can be changed based on the results.
Recently, there has been increased interest and utilization of mono-isocentric delivery techniques. A mono-isocentric technique is one where a single plan with a single isocenter location is used to treat multiple treatment locations within the patient simultaneously. This trend is particularly relevant to stereotactic radiosurgery within the cranium. Utilization of this approach is likely to increase for the foreseeable future due to the desire to spare normal brain tissue as much as possible. The treatment targets may or may not lie directly on the central axis of the beam but in the case of multiple lesions (>2) they generally do not intersect with the isocenter. These “off-axis” fields cannot be easily or quickly measured using current phantom methods needs.
One significant difficulty in verification of mono-isocentric multiple lesion plans arises when a lesion does not lie directly on the isocenter. Many phantoms allow for the placement of measurement detectors in only very specific locations within the phantom body. This significantly limits the variety of tests that may be done.
Currently a multitude of phantoms exist on the market. The most commonly used are commercially available rectangular or cylindrically shaped phantoms. These geometries very poorly approximate the intended target (i.e. a head). Therefore, the interpretation and actual conclusions on the appropriate accuracy of the treatment plan are not truly evaluated. Further issues arise with these phantoms when electronic devices, like diode arrays, are used as the primary detection medium. Because their response is anisotropic and difficult to predict they must be irradiated from a certain direction or corrections must be applied to obtain the correct measurement. In the case of certain phantoms, like those with cylindrical geometry, all the planned therapy beams cannot be delivered as they will be delivered to the patient (i.e. with table “kicks”). This results in an approximate verification of the treatment plan where all possible parameters are not satisfactorily tested. Further, the final error analysis becomes difficult to interpret and little can be done to understand the source of error, if errors are detected.